1. Field of the Invention
This invention relates to a multiple-mode pacer and more specifically, to a pacer which switches from atrial synchronous to a ventricular inhibited mode when the detected intrinsic atrial rate drops below a specified hysteresis rate.
2. Description of the Prior Art
Atrial synchronous pacing is a desirable pacing mode since it most nearly approximates the normal physiological mechanism of the heart. However, such pacers (VDD, DDD, VAT) have not been widely accepted because of the undesirable interaction between the heart and the pacer at the upper and lower rates.
The pacer of the present invention is directed toward a problem exhibited by prior art pacemakers when the detected atrial rate is low. This problem will be described after a discussion of the operation of a normal heart and a brief review of two of the predominate prior art atrial synchronous pacing modes.
In the normal heart the upper chambers or atria contract in a separate action which precedes the contraction of the lower chambers or ventricles. Although the contribution of the atrial contraction is not necessary to sustain life, it does improve the efficiency of the ventricular contraction or systole and aids in the operation of the heart valves. This contraction sequence is normally initiated by electrical activity originating in the sino-atrial (S-A) node. This stimulus is propagated throughout the atrium which contracts. Subsequently, the stimulus is conducted to the atrio-ventricular (A-V) node which delays the delivery of the stimulus to the ventricles for a brief period of time. This (A-V) delay in the heart's conduction system results in the proper temporal relation between the atrial and ventricular contractions of the cardiac cycle.
The electrical activity originating from the cardiac cycle may be recorded at the surface of the body on an electrocardiogram (EKG) which displays a repetitive waveform exhibiting P, Q, R, S, and T wave segments. The P-wave results from the atrial contraction and it is followed by the QRS complex or R-wave portion of the waveform resulting from the ventricular contraction. The time period from the P-wave to the R-wave displayed on the EKG indicates the duration of the A-V delay of the patient's natural conduction system. The T-wave following the R-wave indicates the repolarization or return of the ventricles to the resting state. During this T-wave segment, the heart is particularly vulnerable to life-threatening ventricular fibrillation induced by a pacing stimulus delivered to the ventricle during this T-wave portion of the cardiac cycle. In the normal heart the cardiac cycle repeats at a frequency between 50 and 100 times per minute. On the EKG the heart rate is measured on a beat-to-beat basis by noting the time interval between successive R-waves. For example, the normal resting heart rate of 70 bpm corresponds to an R to R interval of 857 milliseconds.
Atrial synchronous pacers of the VDD and DDD type are indicated for those patients who exhibit reliable intrinsic activity but who have lost A-V synchrony due to a conduction disturbance which interferes with the normal A-V conduction paths. The degree of this atrio-ventricular block indicates the type of atrial synchronous pacer which must be prescribed. The principal atrial synchronous pacing modes include the VAT or atrial synchronous pacer and the VDD or atrial synchronous ventricular inhibited pacer.
A VAT pacer senses the P-wave of the atrium and triggers a ventricular stimulting pulse after an appropriate A-V delay. This delay mimics the normal A-V delay of the heart and restores the proper A-V synchrony between the chambers of the heart.
A VDD pacer senses both P-waves from the atrium and R-waves from the ventricles. In operation, the detected P-wave triggers a ventricular stimulating pulse after an appropriate A-V delay unless the ventricular sense amplifier detects a ventricular contraction during this time period.
Consequently, both of these pacers synchronize the stimulated ventricular event with the intrinsic atrial event which increases cardiac output in some patients and also permits the pacer to follow or track the physiologically determined atrial rate to mimic the natural activity of the normal heart. Thus far these two pacing modalities have been described with respect to the events of a single cardiac cycle.
The VDD pacer, unlike the VAT pacer, provides an inhibit demand pacing function of the ventricles. For example, if ventricular activity occurs during the escape interval, the pacer will not pace in the ventricle until another escape interval expires. At low atrial rates the VDD and VAT pacers revert to a fixed asynchronous ventricular pacing rate.
This stimulation regime can, under certain circumstances, result in the delivery of two closely spaced ventricular stimulating pulses. This is undesirable since the second of these pulses may occur during the vulnerable period of the T-wave at the higher rates. This phenomena may occur if no P-wave is detected during the ventricular escape interval. The absence of this P-wave will result in the asynchronous stimulation of the ventricle. However, if the atria contracts just after the ventricle has been stimulated, then the resultant P-wave will be detected by the pacer; and the pacer will resynchronize on it and deliver another output pulse to the ventricle. These two ventricular stimulating pulses will be separated by a time period corresponding to the upper rate limit characteristic of the pacer.
The prior art teaches two solutions to this problem. The first of these is to increase the A-V delay of the pacemaker such that the second of the two stimulating pulses will not fall within the T-wave. This solution is unsatisfactory since this exaggerated A-V delay reduces the effective pumping efficiency of the heart.
A second prior art solution to this problem is to provide a long refractory period for the atrial sense amplifier. This technique prevents the atrial sense amplifier from sensing atrial activity until a preset delay beyond the delivery of the last stimulating pulse. Although this excessively long refractory period ensures that the pacer will not detect a conducted P-wave, it is undesirable since it may result in missed detection of normal sinus rhythm at high intrinsic rates.